Abstract: The linear relationship between electrical conductivity and thermal conductivity of 36 types of standardized cast Al-Si alloy ingots, which include hypoeutectic alloy, eutectic alloy and hypereutectic alloy, was studied by authoritative measuring of electrical conductivity and thermal conductivity. The results show that the apparent lorentz constant of the cast Al-Si alloy is approximately 1.88×10^-8 WΩ/K² when the Si content varies within the range from 7wt%to 14wt%. This value can be calculated by multiplying the apparent lorentz constant of low alloyed aluminum alloy (2.068×10^-8 WΩ/K²) with the volume percentage of the aluminum phase. The fitted value of lattice thermal conductivity is 35.2 W/ (m·K), which can be calculated by the volume weighted average value of lattice thermal conductivities of low alloyed aluminum and single crystal silicon(Al:16.1 W/(m·K) and Si:140 W/(m·K).The thermal conductivity of Al-Si alloy can be calculated by λ=5.52σ+35.2 W/(m·K), the unit of electrical conductivity(σ) is MS/m.